Electrically heated blanket, guarded against touching the live heater element



y 1. 1967 A. E. F. FICKWEILER 3,330,935

ELECTRICALLY HEATED BLANKET, GUARDED AGAINST TOUCHING THE LIVE HEATER ELEMENT Filed Dec. 24, 1964 United States Patent 3,330,936 ELECTRICALLY HEATED BLANKET, GUARDED AGAINST TOUCHING THE LIVE HEATER ELEMENT August E. F. Fickweiler, Waddinxveen, Netherlands, assignor to Van der Heem N.V., The Hague, Netherlands Filed Dec. 24, 1964, Ser. No. 421,025 Claims priority, application Netherlands, Jan. 15, 1964, 6,400,258 Claims. (Cl. 219-212) ABSTRACT OF THE DISCLOSURE A switch has a switch control electrically connected to a source of heating current via a step down transformer. The switch is controlled by the switch control to selectively open and close electrical conductors connecting a heating wire to the source of heating current. An inner shielding conductor is coaxially wound around the heating wire and separated from the heating wire by electrical insulation. The inner shielding conductor is electrically connected to the switch control by electrical conductor. An outer shielding conductor is coaxially wound around the inner shielding conductor and separated from the inner shielding conductor by electrical insulation. The outer shielding conductor is electrically connected to the switch control by electrical conductor in a manner whereby shortcircuiting of the inner and outer shielding conductors causes the switch control to control the switch to open the electrical conductors and thereby disconnect the heating wire from the source of heating current. A temperature sensitive device is electrically connected between the inner and outer shielding conductors.

The invention relates to an electrically heated blanket, comprising a heater element as well as two measuring conductors, between which one or more temperature sensitive resistors are connected and which in their turn are connected to a switching circuit which disconnects the heater element if the measuring conductors are shortcircuited.

Such a device has been described in the French patent specification 1,344,849. Besides a protection against overheating, which is afforded by the circuit described in the above patent specification, a protection is desired against touching the live heater element, especially if this heater element is directly connected to the mains.

For that purpose, a device according to the application is characterized in that the measuring conductors are arranged coaxially with the heater element and are not directly connected to the mains.

The invention will be further elucidated below with reference to the drawing, which shows an embodiment of a device according to the invention.

FIGURE 1 is a block circuit diagram of a device according to the invention.

FIGURE 2 is a view of the heater and measuring cable for use in a blanket accordingly to the invention, which cable has been peeled off in layers.

In FIGURE 1, the heater wire 1 is through a double pole switch 2 connected to the mains 3. The switch 2 is actuated by a switching circuit 4, as described tag. in the French patent specification 1,344,849, which switching circuit is through an isolation transformer 5 supplied from the mains 3. The switching circuit 4 is controlled by one or more temperature sensitive resistors 6, which are connected to the switching circuit through an outer and an 3,330,936 Patented July 11, 1967 inner conducting sheath 7 and 8 cable.

The insulation between the heater wire 1 and the inner sheath 8 is designed for withstanding the full mains voltage, whilst the insulation between the two sheaths 7 and 8 need be able to withstand only smaller voltages, since this insulation only needs to withstand the small measuring voltage. Around the outer sheath 7, a further thin layer of insulating material is provided, which i.e. serves for mechanically protecting the outer sheath 7.

If for example a metal pin is stuck into the heater and measuring cable, the pin first of all touches the outer sheath 7, which is innocuous, since the measuring circuit is not directely connected to the mains. If the pin is stuck deeper down, the inner sheath 8 too is touched, whereby a short-circuit of the measuring circuit occurs and the heater element is disconnected by means of the switching circuit 4 and the switch 2. If the pin in stuck down still further, it thus touches the heater wire 1 itself, but this is no longer connected with the mains, so that no hazard can arise.

The design of a heater and measuring cable for use in a device according to the invention appears from FIGURE 2. Around a core 9 of insulating fibrous material, a helical winding of resistance wire 10 is provided, which is coated with a layer of high quality insulating material 11. Around the insulating sheath 11, a helical winding of metal tape 12 is provided, which forms inner sheath 8 of FIGURE 1 and which is coated with a layer of insulating material 13, which does not necessarily conform to stringent specifications. Around the insulating sheath 13 a helically wound metal tape 14 is arranged in the opposite direction, which tape is again covered by an outer layer of insulating material. Although for clarity of representation, the metal tapes 12 and 14 have been shown in FIGURE 2 as being wound with some spacing, the tapes can actually be wound closely or even with some overlap, so that it is impossible to reach the heater wire 10 without causing a short-circuit between the sheaths 12 and 14.

Although the above is deemed to be a fair description of the invention, many changes and modifications could be made therein without exceeding the scope of the invention.

What I claim is:

1. A safety heating system for an electric blanket, comprising. a first power source of a first voltage; a second power source of a second safety voltage; an electrical heating element connected to said first power source via switching means; a switching means control device having a power supply input connected to said second power source, and a control input to control said control device in a manner whereby input resistance drop under a predetermined level excites said control device and thereby causes said switching means to open; an inner shielding conductor coaxially arranged around said electrical heating element and separated from that by electrical insulating means, said inner shielding conductor being electrically connected to one pole of control input of said control device; and outer shielding conductor coaxially arranged around said inner shielding conductor and separated from that by electrical insulating means, said outer shielding conductor being electrically connected to the second pole of control input of said control device.

2. The safety heating system according to claim 1 wherein the second power source is formed by a secondary of a step down insulation transformer, the primary of which is connected to said first power source.

respectively of the heater 4 References Cited UNITED STATES PATENTS 2/1957 Lannan et a1. 219-212 X 8/1964 Becker 219-494 FOREIGN PATENTS 1,344,849 10/1963 France.

10 RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Assistant Examiner. 

1. A SAFETY HEATING SYSTEM FOR AN ELECTRIC BLANKET, COMPRISING: A FIRST POWER SOURCE OF A FIRST VOLTAGE; A SECOND POWER SOURCE OF A SECOND SAFETY VOLTAGE; AN ELECTRICAL HEATING ELEMENT CONNECTED TO SAID FIRST POWER SOURCE VIA SWITCHING MEANS; A SWITCHING MEANS CONTROL DEVICE HAVING A POWER SUPPLY INPUT CONNECTED TO SAID SECOND POWER SOURCE, AND A CONTROL INPUT TO CONTROL SAID CONTROL DEVICE IN A MANNER WHEREBY INPUT RESISTANCE DROP UNDER A PREDETERMINED LEVEL EXCITES SAID CONTROL DEVICE AND THEREBY CAUSES SAID SWITCHING MEANS TO OPEN; AN INNER SHIELDING CONDUCTOR COAXIALLY ARRANGED AROUND SAID ELECTRICAL HEATING ELEMENT AND SEPARATED FROM THAT BY ELECTRICAL INSULATING MEANS, SAID INNER SHIELDING CONDUCTOR BEING ELECTRICALLY CONNECTED TO ONE POLE OF CONTROL INPUT OF SAID CONTROL DEVICE; AND OUTER SHIELDING CONDUCTOR COAXIALLY ARRANGED AROUND SAID INNER SHIELDING CONDUCTOR AND SEPARATED FROM THAT BY ELECTRICAL INSULATING MEANS, SAID OUTER SHIELDING CONDUCTOR BEING ELECTRICALLY CONNECTED TO THE SECOND POLE OF CONTROL INPUT OF SAID CONTROL DEVICE. 